In a wind farm as renewable energy power station, the wind power generators as renewable energy sources are connected in several stages to a high voltage DC distribution grid. In a first, medium voltage level stage, each renewable energy source may be connected via an indirect AC-to-AC converter to a distribution bus. The wind generator is usually driven in a variable speed operation mode, which allows a higher energy output, lower mechanical stress and less power fluctuation when compared to a system operating with constant speed. In a second, high voltage stage, one or more of the distribution buses are connected via an AC-to-DC converter with the high voltage DC distribution grid.
The main functionalities of a converter interconnecting a wind power generator with the distribution bus are the regulation of the output of the wind power generator and at the same time the energy conversion from variable frequency and voltage to constant frequency and constant voltage.
Usually the first converter comprises an energy source side AC-to-DC converter interconnected via a DC link with a bus side DC-to-AC converter. The control strategies for these converters may differ according to the application. For example, in strong grid applications, where the contribution of the wind energy into the distribution bus is small compared to the total installed power, the bus side converter is used primarily to regulate the DC link voltage to the optimal level for the energy source side rectifier. At the same time, the current injected into the distribution bus has to meet standards related to harmonic distortion and power quality. For example, a three-level converter may be used to perform such a task. The energy source side AC-to-DC converter regulates the speed of the wind power generator (for example a PMG) to achieve the desired power transfer for the given wind conditions. In addition, the AC-to-DC converter can achieve the optimal operating point for the system, considering that at a given wind speed, the maximum turbine energy conversion efficiency occurs at an optimal ratio of the turbine rotor tip speed to the wind speed. This means that as the wind speed changes, the rotor speed of the turbine must change accordingly to maintain the optimal turbine energy conversion ratio.
On the other hand, in applications where the contribution of the wind energy is a significant portion of the installed power and the energy is fed to a weak grid, the functionalities of the converters differ. In this case, the main goal of the bus side converter is to regulate the frequency and voltage of the distribution bus. The DC link voltage is controlled by the energy source side converter.
Between the AC-to-AC converter interconnecting a renewable energy source and the distribution grid, a low frequency transformer may be used for stepping up the voltage. Another transformer may be used to increase the medium AC voltage in the distribution bus to a high voltage to be input into the AC-to-DC converter before the high voltage DC distribution grid, which rectifies the AC voltage and controls the DC voltage to be input into the high voltage DC distribution grid.
Such a converter system may present disadvantages such as the need for several conversion stages which results in lower efficiency, higher costs and difficulty to comply with grid code standards. The use of low frequency transformers may be another disadvantage, since it penalizes weight, size and power density (which may be some of the key parameters in offshore applications).
Alternatively, the AC distribution bus may be replaced with a medium voltage DC distribution bus. In this DC approach, the resulting AC voltage from the wind power generator is rectified and stepped up to the DC medium voltage. Several such converter units may be interconnected to one DC distribution bus. Another DC-to-DC converter may then be used for the connection to the high voltage DC distribution grid. The main benefit of such an approach may be the simplification in the connection of several converter units to the DC distribution bus and the advantage of connecting a medium voltage DC distribution bus to a high voltage DC distribution grid. However, this approach still presents some drawbacks such as the need for fast circuit breakers and current limiters which protect the converter units connected to the wind power generators under short circuit and over current conditions. Furthermore, the DC-to-DC converter usually also comprises several cascaded isolated DC-to-DC converter stages.
For example, EP 2 341 594 A1 describes a power collection and transmission system, which uses isolated DC-to-DC converters for connecting wind power generators with a high voltage distribution grid in two isolated stages.
WO 2011/029566 A1 shows a further example for a two stage system with intermediate DC distribution bus for supplying electrical loads.
U.S. 2010/0156189 A1 relates to collection of electric power from renewable energy sources and shows a further example how isolated DC-to-DC converters may be used for connecting wind power generators to a distribution grid.